1. Field of the Invention
The present invention relates to a servo control apparatus for controlling a subject to be controlled (i.e., controlled system) defined by a characteristics equation including at least a second order term, a first order term and a zero order term by the feedback control, more specifically, it relates to a servo control apparatus for estimating the disturbance to be applied on the controlled system and compensating an actual disturbance on the basis of the estimated disturbance.
2. Description of the Related Art
Recently, studies have been conducted actively on the technique for preliminarily estimating the disturbance to be applied on the controlled system in the feedback control of a controlled system, and compensating an actual disturbance by applying the manipulated variable considering the estimated disturbance to the controlled system.
As a preferable device for predicting the disturbance, a so-called state observer is attracting the attention recently.
The observer will be explained.
The observer is a device for estimating the state that cannot be detected actually from the detectable state. The state that cannot be detected actually is, for example, the state of a controlled system with a disturbance applied. The disturbance predicted to be applied on the controlled system is estimated by the observer so that the manipulated variable to be corrected is calculated based on the estimated disturbance variable, and the result thereof is added to the manipulated variable in the above-mentioned feedback control system. Accordingly, the disturbance actually applied to the controlled system is compensated.
The process for estimating the disturbance in the observer will be explained with reference to FIG. 12. FIG. 12 shows the case of adopting the observer to a focus servo control system for a focus actuator included in an information reproducing apparatus such as CD (Compact Disk) player as the controlled system, specifically it shows a feedback servo loop formed in the focus servo control system.
In the information reproducing apparatus, information recorded on an optical disk is reproduced by irradiating the information recording surface of the optical disk with a light beam. In this process, the focus actuator is used for setting a focal position of the light beam. The focus actuator is an actuator for moving an objective lens in the direction perpendicular to the information recording surface of the optical disk to focus the light beam on the information recording surface.
The focus actuator supports the objective lens by an elastic member such as a plate spring. A characteristics equation showing the controlled system having such a configuration, in general, includes at least a second order term, a first order term and a zero order term. Moreover, the characteristics equation can include also a term of three order or more. Hereinafter, such a controlled system will be referred to as a spring-mass system.
In FIG. 12, the controlled system U(s) is the focus actuator, and the controlled variable y is the position of the objective lens to be moved by the actuator.
The characteristics equation (transfer function) of the actuator can be represented in the second order delay system as:
U(s)=Axc3x97wa2/(s2+2xc3x97kaxc3x97waxc3x97s+wa2)xe2x80x83xe2x80x83(1).
Herein, A is the gain of the actuator (m/Ampere), ka is the viscosity braking coefficient of the actuator, and wa is the natural frequency of the actuator (rad/sec).
With the conversion sensitivity for outputting the focus error signal in the actuator (that is, the conversion sensitivity determined by the sensitivity of the photo detector in the information reproducing apparatus and the amplifying ratio of the error generation amplifier) considered to be the position detecting sensitivity Ke (volt/m), it can be represented as:
REFxe2x88x92yxc3x97Ke=erxe2x80x83xe2x80x83(2).
Herein, REF is the desired value wherein the objective lens should be disposed, and er is the error in the above-mentioned feedback control system. As shown in FIG. 12, the error er obtained by the formula (2) is input to one of the input terminals of the observer.
On the other hand, the relationship between the manipulated variable (voltage value) u and the drive current i for driving the actuator in FIG. 12 can be represented as:
i=Kdrxc3x97uxe2x80x83xe2x80x83(3).
Herein, Kdr (Ampere/Volt) is the voltage/current conversion sensitivity of the driver (to be driven by the manipulated variable u) for generating the drive current i. The drive current i is converted to the input voltage v by the current/voltage converter with the current/voltage conversion sensitivity Kiv (Volt/Ampere) so as to be input to the other input terminal of the observer as shown by the following formula (4):
v=Kivxc3x97Ixe2x80x83xe2x80x83(4).
Herein, the above-mentioned current/voltage conversion sensitivity Kiv corresponds to the conversion sensitivity in feeding back the drive current i to the observer, that is, so-called return resistance.
In order to simplify the explanation, the disturbance at a certain position is considered to be the only disturbance to be applied on the actuator. As shown in FIG. 12, with the disturbance variable defined as d, it can be represented as:
ixc3x97U(s)+d=yxe2x80x83xe2x80x83(5).
Herein, in the case the desired value REF is zero (REF=0) in the above-mentioned formula (2), it can be represented as:
yxc3x97Ke=xe2x88x92er.
Therefore, from the above-mentioned formula (4), it can be represented as:
i=v/Kiv.
By erasing i and y from the above-mentioned formula (5), it can be represented as:
(v/Kiv)xc3x97U(s)+d=xe2x88x92er/Ke.
By rearranging the formula, the disturbance variable d can be represented as the following formula (6) by using the input voltage v to be input to the observer corresponding to the drive current i and the error er:
d=xe2x88x92er/Kexe2x88x92(v/Kiv)xc3x97U(s)xe2x80x83xe2x80x83(6).
Herein, the parameters showing the inside of the observer as a model is represented as nominal values. In order to distinguish this from the actual control element, it is shown with an additional letter n. That is, the position detecting sensitivity Ke is represented as the position detecting sensitivity nominal value Ken, the voltage/current conversion sensitivity Kdr is represented as the voltage/current conversion sensitivity nominal value Kdrn, the current/voltage conversion sensitivity Kiv is represented as the current/voltage conversion sensitivity nominal value Kivn, and the controlled system U(s) is represented as the controlled system nominal value Un(s). The nominal value corresponding to the controlled system may be referred to, in general, as the internal model of the observer.
Incidentally, the nominal value is, for example, the torque rated value of the spindle motor for rotating the optical disk in an information reproducing apparatus, which is the value shown in the performance indication of the information reproducing apparatus, or the like. In the case, the performance indication is not provided, it is determined preliminarily by an experiment, or calculated by the theoretical calculation. The nominal value and the actual control element cannot always be equal due to factors such as insufficient accuracy of the determination or the calculation, aging, and temperature change.
From the above-mentioned formula (6), the estimated disturbance variable DOBS, which is the estimated variable of the disturbance d can be represented using the nominal values as:
DOBS=xe2x88x92er/Kenxe2x88x92(v/Kivn)xc3x97Un(s)xe2x80x83xe2x80x83(7).
Accordingly, the estimated disturbance variable DOBS can be calculated from the input voltage v and the error er using the observer without detecting the actual disturbance d.
In FIG. 12, the disturbance d is suppressed by generating the manipulated variable u by multiplying the calculated estimated disturbance variable DOBS by the inverse transmission characteristics from the manipulated variable u to the controlled variable y (1/{Kdrnxc3x97Un(s)}), further, converting the same into the compensated variable h by the robust filter R(s), and adding the compensated variable h to the variable obtained by applying the phase compensation to the error er by the phase compensator C(s).
In the case an information reproducing apparatus, or the like, including the above-mentioned focus actuator is provided for the public use, it is preferable to provide the observer with a simple configuration, and consequently, to reduce the production cost and the price of the product.
For example, in the case the observer is provided using a so-called DSP (Digital Signal Processor), the word length is preferably 16 bit rather than 32 bit for the public use. Moreover, the processing method is preferably the fixed point type rather than the floating point type for the public use.
However, in order to estimate the disturbance using the internal model corresponding to the controlled system of the spring-mass system by an observer with such a simple configuration, long estimating time is needed because the characteristics equation defining the internal model includes at least a second order term, a first order term and a zero order term as mentioned above. As a result, a problem is involved in an optical disk apparatus requiring an actuator with a high response frequency in that the process for estimating the disturbance cannot follow the process of the actuator, and thus the disturbance cannot be restrained effectively.
Moreover, in the case the disturbance is estimated by an observer with a simple configuration by using the internal model corresponding to the controlled system of the spring-mass system, much error can be included in the estimation result derived from the low processing ability of the observer itself. As a result, a problem arises in that the disturbance cannot be restrained with a high accuracy.
An object of the present invention is to provide a servo control apparatus with simple configuration at a low cost, capable of a feedback control to a controlled system while estimating a disturbance applied to the controlled system accurately and quickly.
A servo control apparatus in accordance with the present invention performs a feedback control to a controlled system having characteristics represented by a characteristic equation including at least a second order term, a first order term and a zero order term. The servo control apparatus includes a disturbance estimating device that has an internal model having characteristics represented by a characteristic equation including only a second order term, and that estimates a disturbance applied to the controlled system by using the internal model, thereby generating an estimated disturbance value; and a compensating device that compensates the disturbance on the basis of the estimated disturbance value.
In the servo control apparatus of the present invention, a disturbance applied to the controlled system having characteristics represented by a characteristic equation including at least a second order term, a first order term and a zero order term is estimated by using the internal model of the disturbance estimating device having characteristics represented by a characteristic equation including only a second order term. Although the characteristic equation of the controlled system does not completely coincide with that of the internal model of the disturbance estimating device, the disturbance can be estimated. Especially, it can be accurately estimated at least in a frequency band higher than the natural frequency of the controlled system. Furthermore, the characteristic equation of the internal model of the disturbance estimating device is simpler than that of the controlled system. Therefore, the disturbance can be easily and quickly estimated without the deterioration of the accuracy of the disturbance estimation, by using the disturbance estimating device with a simple structure.
In the servo control apparatus in accordance with the present invention, the internal model may include at least two integration elements, and at least one of the integration elements may be of an incomplete integration type. By using the integration element of the incomplete integration type, the saturation of the integration element can be prevented, so that the transitional characteristics of the feedback control can be improved, at least as compared with an internal model in which all of the integration elements are of a complete integration type.
In the servo control apparatus in accordance with the present invention, the disturbance estimating device may include a robust filtering device having second or more order low-pass filtering characteristics. According to the robust filtering device, the deterioration of the accuracy of the disturbance estimation due to the incompleteness in the internal mode or the controlled system, the observation noise, or the like can be prevented.
In the servo control apparatus in accordance with the present invention, the disturbance estimating device may includes a digital processing unit for estimating the disturbance by using a digital manipulated variable at a current sample timing, a digital error at the current sample timing and a digital state variable at a previous sample timing. According to such a digital configuration, the accuracy of the disturbance estimation can be improved. Furthermore, the disturbance estimating device can be realized as a DSP.
Another object of the present invention is to provide a servo control method capable of a feedback control to a controlled system with a simple configuration while estimating a disturbance applied to the controlled system accurately and quickly.
A servo control method in accordance with the present invention is a method for a feedback control to a controlled system having characteristics represented by a characteristic equation including at least a second order term, a first order term and a zero order term. The method includes the processes of estimating a disturbance applied to the controlled system by using the internal model having characteristics represented by a characteristic equation including only a second order term, thereby generating an estimated disturbance value; and compensating the disturbance on the basis of the estimated disturbance value.
The nature, utility, and further feature of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiments of the invention when read in conjunction with the accompanying drawings briefly described below.